Frequency Modulation Questions and Answers

Q: What deviation does FM broadcasting use?

The standard is plus or minus 75 kHz, giving a deviation ratio of five against a 15 kHz audio limit. Five kilohertz is narrowband FM for voice. Two hundred kilohertz is the channel spacing rather than the deviation. And fifteen kilohertz is the audio bandwidth. Confusing deviation with channel spacing is easy and consequential, since the two differ by more than a factor of two once Carson's rule is applied.

Q: Which detector is inherently amplitude-insensitive once locked?

A phase-locked loop tracks frequency and its output depends on the oscillator's control voltage rather than on input amplitude, so it is largely immune once captured. The three alternatives all convert frequency to amplitude at some stage and are therefore sensitive to it, needing a limiter ahead of them. That practical difference, along with its integrability, is why the loop dominates modern FM receivers.

Q: What does the deviation ratio describe?

It is the index evaluated at the highest message frequency, which is the worst case for bandwidth and therefore the figure a system is designed around. The lowest message frequency gives the largest index but the narrowest spectrum in Carson terms, so it is not the design case. Channel spacing is a regulatory quantity. And FM power ratios are not what the term describes. It is a system parameter where the modulation index is a per-tone one.

Q: Which is constant in an FM signal?

Only the amplitude is constant. The instantaneous frequency varies with the message, which is the definition. The bandwidth varies with deviation and hence with modulation depth. And the index varies with both deviation and message frequency, so it differs for every tone in a complex message. Identifying the one invariant is the quickest route to answering questions about power, amplifier class and limiter action.

Q: Which modulation is produced by differentiating the message before a frequency modulator?

Differentiating before an FM modulator gives phase modulation, since frequency is the derivative of phase and the two operations cancel. The dual arrangement, integrating before a phase modulator, gives FM. Amplitude modulation needs an entirely different modulator. Single sideband is an amplitude scheme. And no pre-processing simply doubles the index. The pair of equivalences is why the two schemes are studied as one family.

Q: What limits the deviation an Armstrong transmitter can achieve at the phase modulator?

A practical phase modulator is linear only over a small phase excursion, so the index produced is small and frequency multiplication is used to raise it to what broadcasting needs. That constraint is the whole reason the multiplier chain exists. Crystal tolerance affects centre frequency rather than deviation. Audio gain sets the drive but not the linear range. And antenna bandwidth is a matching matter far downstream.

Q: Why does frequency multiplication not increase the message frequency?

Multiplying scales the radio-frequency waveform, so the carrier and the deviation both grow while the rate at which the deviation swings, set by the audio, is unchanged. The index therefore rises by the multiplication factor. The audio is not removed and not reinserted; it is carried through as deviation. And a multiplier is deliberately nonlinear, which is how it generates harmonics. This asymmetry between what scales and what does not is the key to the Armstrong method.

Q: A phase modulator gives an index of \( 0.5 \) at \( 100 \ \mathrm{kHz} \). After multiplication by 64, the index becomes

Multiplication scales the index with the frequency:\[ \beta = 0.5 \times 64 = 32 \]so 32, at a carrier of 6.4 MHz. The index staying at 0.5 would make the method pointless. 64 is the multiplication factor alone. And 128 doubles it. This is exactly how a stable crystal source is turned into a wideband FM signal, and it shows why the multiplier chain is a defining feature of the indirect method.

Q: What does automatic frequency control correct in a direct FM transmitter?

The loop holds the centre frequency against drift, comparing the oscillator with a crystal reference through a long time constant that does not fight the wanted modulation. Deviation is set by the modulator's sensitivity and is checked separately. Audio distortion is a linearity matter. And antenna mismatch is corrected by tuning the output network. Direct FM's simplicity is bought at the price of needing this loop, which is the trade against the Armstrong method.

Q: Which is the correct expression for instantaneous frequency in FM?

The instantaneous frequency is the carrier plus a deviation proportional to the message amplitude. Multiplication would produce a quite different and unusable signal. A term in the message's derivative describes phase modulation written as a frequency deviation, which is the dual case. And dividing by the index is meaningless. Writing the expression down correctly is the starting point for deriving the index, and hence the spectrum, of any FM signal.

61.

What deviation does FM broadcasting use?

A.Plus or minus 75 kHz
B.Plus or minus 5 kHz
C.Plus or minus 200 kHz
D.Plus or minus 15 kHz

Answer: A

The standard is plus or minus 75 kHz, giving a deviation ratio of five against a 15 kHz audio limit. Five kilohertz is narrowband FM for voice. Two hundred kilohertz is the channel spacing rather than the deviation. And fifteen kilohertz is the audio bandwidth. Confusing deviation with channel spacing is easy and consequential, since the two differ by more than a factor of two once Carson's rule is applied.

Discuss in Forum

62.

Which detector is inherently amplitude-insensitive once locked?

A.The balanced slope detector
B.The phase-locked loop
C.The slope detector
D.The Foster-Seeley discriminator

Answer: B

A phase-locked loop tracks frequency and its output depends on the oscillator's control voltage rather than on input amplitude, so it is largely immune once captured. The three alternatives all convert frequency to amplitude at some stage and are therefore sensitive to it, needing a limiter ahead of them. That practical difference, along with its integrability, is why the loop dominates modern FM receivers.

Discuss in Forum

63.

What does the deviation ratio describe?

A.The ratio of deviation to channel spacing
B.The ratio of carrier power to sideband power
C.The index at the highest message frequency
D.The index at the lowest message frequency

Answer: C

It is the index evaluated at the highest message frequency, which is the worst case for bandwidth and therefore the figure a system is designed around. The lowest message frequency gives the largest index but the narrowest spectrum in Carson terms, so it is not the design case. Channel spacing is a regulatory quantity. And FM power ratios are not what the term describes. It is a system parameter where the modulation index is a per-tone one.

Discuss in Forum

64.

Which is constant in an FM signal?

A.Instantaneous frequency
B.Occupied bandwidth
C.Modulation index
D.Amplitude

Answer: D

Only the amplitude is constant. The instantaneous frequency varies with the message, which is the definition. The bandwidth varies with deviation and hence with modulation depth. And the index varies with both deviation and message frequency, so it differs for every tone in a complex message. Identifying the one invariant is the quickest route to answering questions about power, amplifier class and limiter action.

Discuss in Forum

65.

Which modulation is produced by differentiating the message before a frequency modulator?

A.Phase modulation
B.Amplitude modulation
C.Single sideband
D.Wideband frequency modulation of double the index

Answer: A

Differentiating before an FM modulator gives phase modulation, since frequency is the derivative of phase and the two operations cancel. The dual arrangement, integrating before a phase modulator, gives FM. Amplitude modulation needs an entirely different modulator. Single sideband is an amplitude scheme. And no pre-processing simply doubles the index. The pair of equivalences is why the two schemes are studied as one family.

Discuss in Forum

66.

What limits the deviation an Armstrong transmitter can achieve at the phase modulator?

A.The antenna's bandwidth
B.Phase modulator linearity at small angles
C.The crystal's frequency tolerance
D.The audio amplifier's gain

Answer: B

A practical phase modulator is linear only over a small phase excursion, so the index produced is small and frequency multiplication is used to raise it to what broadcasting needs. That constraint is the whole reason the multiplier chain exists. Crystal tolerance affects centre frequency rather than deviation. Audio gain sets the drive but not the linear range. And antenna bandwidth is a matching matter far downstream.

Discuss in Forum

67.

Why does frequency multiplication not increase the message frequency?

A.The multiplier is a linear device
B.The audio is reinserted after multiplication
C.Multiplication scales the carrier and its deviation, not the audio
D.The audio is removed before multiplication

Answer: C

Multiplying scales the radio-frequency waveform, so the carrier and the deviation both grow while the rate at which the deviation swings, set by the audio, is unchanged. The index therefore rises by the multiplication factor. The audio is not removed and not reinserted; it is carried through as deviation. And a multiplier is deliberately nonlinear, which is how it generates harmonics. This asymmetry between what scales and what does not is the key to the Armstrong method.

Discuss in Forum

68.

A phase modulator gives an index of \( 0.5 \) at \( 100 \ \mathrm{kHz} \). After multiplication by 64, the index becomes

A.0.5
B.64
C.128
D.32

Answer: D

Multiplication scales the index with the frequency:

\[ \beta = 0.5 \times 64 = 32 \]

so 32, at a carrier of 6.4 MHz. The index staying at 0.5 would make the method pointless. 64 is the multiplication factor alone. And 128 doubles it. This is exactly how a stable crystal source is turned into a wideband FM signal, and it shows why the multiplier chain is a defining feature of the indirect method.

Discuss in Forum

69.

What does automatic frequency control correct in a direct FM transmitter?

A.Drift of the oscillator's centre frequency
B.Errors in the deviation
C.Distortion in the audio chain
D.Mismatch at the antenna

Answer: A

The loop holds the centre frequency against drift, comparing the oscillator with a crystal reference through a long time constant that does not fight the wanted modulation. Deviation is set by the modulator's sensitivity and is checked separately. Audio distortion is a linearity matter. And antenna mismatch is corrected by tuning the output network. Direct FM's simplicity is bought at the price of needing this loop, which is the trade against the Armstrong method.

Discuss in Forum

70.

Which is the correct expression for instantaneous frequency in FM?

A.Carrier frequency divided by the modulation index
B.Carrier frequency plus a term proportional to the message
C.Carrier frequency multiplied by the message
D.Carrier frequency plus a term proportional to the message's derivative

Answer: B

The instantaneous frequency is the carrier plus a deviation proportional to the message amplitude. Multiplication would produce a quite different and unusable signal. A term in the message's derivative describes phase modulation written as a frequency deviation, which is the dual case. And dividing by the index is meaningless. Writing the expression down correctly is the starting point for deriving the index, and hence the spectrum, of any FM signal.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Frequency Modulation Questions and Answers || Frequency Modulation MCQ Questions and Answers || Frequency Modulation GK Questions and Answers || Frequency Modulation GK MCQ Questions || Frequency Modulation Multiple Choice Questions and Answers || Frequency Modulation GK || GK on Frequency Modulation || Analog Communication Questions and Answers || Analog Communication MCQ Questions and Answers || Analog Communication GK Questions and Answers || GK on Analog Communication

Frequency Modulation Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications